To start, we assume excess SiO2, H2O, meaning that quartz/coesite and H2O are stable in all mineral assemblages.

Then we assume that plagioclase is also stable and of an unknown, but unimportant, composition; by this we ignore the effects of Na and Ca.

This results in a tetrahedron of A=Al2O3, K=K2O, F=FeO, M=MgO:

To render this diagram in easier-to-understand-and-draw 2D, we project the compositions in the tetrahedron from muscovite KAl3Si3O10(OH)2 onto the AFM plane

This results in a common pelitic assemblage such as st + gar + bio + mus + plg + qtz + H2O (7 phases) plotting as a triangle. This is convenient, and possible only because we have assumed that i) qtz + H2O are in excess, ii) plagioclase is present, and iii) muscovite of fixed composition is present.

Metamorphic Zones

Barrovian metamorphic zones are defined by reactions that result in the appearance or disappearance of minerals and can be mapped as isograds
chl > bi > gar > st > ky > sill > sill + or

Barrovian zone

mineral assemblage

chlorite zone

chlorite + mus + qtz + H2O + relict minerals

biotite zone

chlorite + biotite + mus + qtz + H2O

garnet zone

chlorite + biotite + garnet + mus + qtz + H2O

staurolite zone

staurolite + 2 AKFM phases + mus + qtz + H2O

kyanite zone

kyanite + 2 AKFM phases + mus + qtz + H2O

sillimanite zone

garnet + biotite + sillimanite + mus + qtz + H2O

2nd sillimanite zone or
sillimanite + orthoclase zone

sill + or + qtz + H2O + melt and no mus

This development of metamorphic mineral assemblages corresponds to this P-T path:

Below is an example showing the prograde metamorphism of pelites around the Kangmar Dome of Tibet.

View the AFM movies #1 and #2 of Worley and Powell showing how mineral compositions and stabilities can change with changing temperature.